1. Field of the Invention
The present invention relates to a method and apparatus for easily implementing a multi-wavelength light source the frequency intervals of which are equal.
2. Description of the Related Art
The wavelength of signal light in a WDM (Wavelength Division Multiplexing) optical fiber communications system is stipulated to be arranged on a predetermined frequency grid by the ITU-T recommendations. Therefore, an absolute wavelength must be precisely controlled for an oscillation on this grid.
For a method preparing single wavelength lasers by a required number of channels, firstly, its monitoring/control becomes complicated. Secondly, it is inevitable to increase the size and the power consumption of an apparatus if the number of wavelengths, namely, the number of channels becomes large.
As a method for solving these problems, there is a method for splitting longitudinal mode components caused by modulation, and making the components into a multi-wavelength light source (see Non-Patent Document 1). The longitudinal mode is a spectrum component caused by modulation. If the spectrum of modulated light is viewed with a spectrum analyzer having a low resolution, it is shaped like a moderate mountain. However, if the spectrum is viewed with a spectrum analyzer having a high resolution, it is proved to be actually composed of many spectrum components having a narrow spectrum width. Each spectrum component having a narrow spectrum width, which configures such a spectrum of modulation light, is called a longitudinal mode component.
In FIG. 1, an optical pulse sequence from a pulse light source 10 that outputs an optical pulse sequence of a repetitive frequency f0 Hz is input to a modulator array 11. In a wavelength demutiplexer 11-1, the longitudinal mode components of the optical pulse sequence are split and made into light beams having respective wavelengths. Then, the light beams are modulated by a modulator 11-3, and signals are put on the modulated light beams. Thereafter, these modulated light beams are coupled by a wavelength multiplexer 11-2, and transmitted.
Another characteristic of this method exists in a point that the number of channels can be increased by using spectrum broadening caused by nonlinear effects that occur within a nonlinear medium.
As conventional multi-wavelength light sources, techniques recited in Patent Documents 1 and 2 exist. With the technique recited in Patent Document 1, a longitudinal mode component obtained from modulated light is demultiplexed, and made into a light source of each wavelength. Patent Document 2 discloses the technique with which an optical pulse sequence from a light source that generates an optical pulse sequence is passed through an optical fiber the dispersion of which is flattened to widen the width of a spectrum by nonlinear effects, and a longitudinal component is extracted from the widened spectrum.
[Patent Document 1]
Japanese Patent Application Publication No. 2001-264830
[Patent Document 2]
Japanese Patent Application Publication No. 2002-236301
[Non-Patent Document]
IEEE Photonics Technology Letters, Vol. 9, No. 6, June 1997, pp. 818-820
FIGS. 2A and 2B exemplify the configuration of an optical transmitting apparatus using another conventional multi-wavelength light source, and the shape of a spectrum.
In FIG. 2A, a pulse light source 15 outputs an optical pulse sequence of a repetitive frequency f0 Hz, a spectrum expanding device 16 expands the spectrum of the optical pulse sequence, a modulation array then modulates each longitudinal mode component, and a gain equalizer 18 realizes the same power of each wavelength.
A plurality of single wavelength light sources can be created by extracting the longitudinal mode components of an optical spectrum with a narrow band filter as described above. With the conventional technique shown in FIG. 2A, however, the flatness of the spectrum of light after being broadened is poor, and the powers of respective signal wavelengths significantly vary.
A spectrum broadened by using a Gaussian pulse is exemplified in FIG. 2B. A horizontal axis represents a wavelength, whereas a vertical axis represents power on a linear scale. This spectrum is shaped like an envelope that connects the peaks of longitudinal mode components. Namely, this figure shows the spectrum viewed with a spectrum analyzer having a low resolution. As is known from this spectrum, powers vary by wavelength on the order of several times. Accordingly, it is difficult to create a practical multi-wavelength light source. To actually apply this multi-wavelength light source to a WDM communications system, a gain equalizer that equalizes a power difference among channels must be incorporated. The gain equalizer matches the powers of wavelengths with that of a wavelength having the lowest power. Therefore, light of a wavelength originally having high power is attenuated, so that the loss of optical power increases, and also an optical signal to noise ratio is degraded.